1. Field of the Invention
The invention relates to a discharge lamp in which there are a pair of electrodes in an arc tube.
2. Description of Related Art
As light sources for general illumination, industrial endoscopes, overhead projectors, and liquid crystal back lights, metal halide lamps are used which are a type of discharge lamp.
In the following, a process for production of a conventional metal halide lamp is described using FIG. 5.
(Process of electrode machining)
Electrodes of tungsten or the like are machined by means of a lathe or the like. In doing so, tips and parts of the electrode to which metal foils are later welded are shaped. In the drawing, an electrode 1 has been given a conically shaped tip 100 while a part 110 at the opposite end of the electrode which has had a flat shaped thereon.
(Process of electrode treatment)
Electrodes 1 machined in the above described manner are subsequently subjected to electrolytic polishing and cleaning, and furthermore, are degassed in a vacuum heating furnace.
(Process of attaching mounts)
To one end of molybdenum foil 2, an outer lead 3 is welded, while to the opposite end of the foil 2, the electrode 1, which has been treated in the above described manner, is welded. An assembly comprised of electrode 1, molybdenum foil 2 and outer lead 3 is called a "mount," and in the drawing, the mount is labelled with reference letter C.
(Process of production of an arc tube)
An arc tube of fused silica glass 4 has a bulb essentially in its middle, this bulb being intended to form the emission space.
(Process of insertion of the mount)
Mounts C are inserted into arc tube 4 and located at a distance to one another in order to obtain the desired distance between the electrodes.
(Process of sealing)
Next, arc tube 4 is turned. Heat is supplied to the outside of the regions provided with the molybdenum foils 2, and thus, hermetic sealing is obtained. In doing so, at the same time, control of the distance between the electrodes, the eccentricity of the electrodes and the like, is performed using a magnifier or CCD camera. After completion of hermetic sealing, the length of the distance between the completed electrodes is recorded.
(Process of filling)
Next, filler is added to arc tube 4 through a tube 5. Fillers are, for example, halides in the form of a pellet, for example, SnI.sub.2. Here, it is necessary that the amount of filler is matched to the length of the distance between the electrodes so that the electrical characteristic is always constant. After adding the filler, outlet tube 5 is sealed and thus the lamp is completed.
However this production process has the following disadvantages:
The sealing process is performed manually using a lathe or the like to obtain an exact distance between the electrodes. This process is performed using a magnifier, CCD camera or the like such that the distance is visually matched. It is therefore difficult to always achieve an exact distance. The sealing process is furthermore performed such that the arc tube is turned around its longitudinal direction. It is therefore necessary to control the eccentricity of the electrodes. In reality, therefore, the lamps have small deviations in the distance between the electrodes. The corresponding amount of filler to be added can, therefore, only be adjusted after measuring the distance between the electrodes in the respective lamp. In particular, recently the distance between the electrodes has become extremely short, i.e. it is no more than 3.0 mm. The indicated problem therefore becomes particularly pronounced in such situations.
Furthermore, there is the disadvantage that, in this production process, neither mass production nor automation are possible.
To eliminate these disadvantages the present inventor has already devised an invention which has been disclosed in Japanese patent disclosure document HEI 6-310030.
In this production process, hermetic sealing of the mounts is performed in the state in which the two electrodes are joined to one another. Afterwards, through a tube (hereinafter also called filling tube), a rod is inserted. With this rod a slot is punched which has been arranged beforehand on the upholding parts of the electrode connected to one another. Thus, the distance between the electrodes is adjusted. Based on this technology, the distance between the electrodes is produced after the mounts are sealed. Therefore, an exact distance between these electrodes can always be established.
However, in the process which is disclosed in the above described prior art, it is necessary for producing the distance between the electrodes to insert a rod through a tube formed on the bulb of the arc tube. Therefore, on the outside surface of the arc tube of the completed lamp, there necessarily remains a residual part of the tube (generally called a tip).
Recently, because the emission space has become smaller and smaller, the effects of the tip have therefore become less and less negligible. Specifically, due to the presence of the tip, the effective radiation surface on the outside surface of the lamp is accordingly reduced. Furthermore, scattered light which is formed by the tip is emitted in the form of undesirable radiation onto the screen and the like.